Colors, vision and polarized light

If we are asked to choose between a film in black and white, and a similar one, but in colors, there is no doubt that we will all lean towards the latter. And it is not because we think that it is closer to reality, but that colors attract us because of the beauty they arouse. The same could be said of a painting, a sheet, and even the day. Our admiration for a sunny day is due to the great variety of colors that such a day reveals to us. On the other hand, when it is cloudy we can not distinguish the nuances so clearly and everything appears opaque and gray to our eyes. Light is the great responsible for colors; but a complete study of the problem also requires knowledge of other phenomena of no less importance.

In fact, in order to understand the question of the vision of colors, the participation of three branches of science is necessary: ​​physics, physiology and psychology. It corresponds to each one the study of the light, the eye and the psychism, respectively.

A prism of glass can look like a simple geometric body. However, since Newton discovered that with it it is possible to decompose white light into various colors, it became a very useful instrument for physics. And we can say that there began the study of colors.

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When a ray of sunlight passes through a prism, it decomposes into a series of colors, of which seven can easily be distinguished: violet, indigo, blue, green, yellow, orange and red. Newton, after discovering this fact, proved that, conversely, once again mixing all these colors, the white reproduced again. What does it mean, then, that we see a body of a given color? Let, for example, be an object of red color; we see it with said coloration because only the red light reflected by the body reaches our eyes. And what happened to the light of the other colors? Well, it has been absorbed by the object. According to this, a color will be more pure the less the presence of other colors. In other words, painting a blue wall, for example, means having the wall absorb all the colors and reflect the blue.

Newton's investigations do not end here; he also discovered the existence of primary or fundamental colors: red, yellow and blue, by means of which it is possible to obtain the others.

It took a long time for Newton's ideas to be criticized. But such criticisms were made and nothing less than by the German poet John W. Goethe, who claimed that the vision of color contributed not only light but also darkness. Thus, Goethe argued that color should be the result of an intimate relationship between two opposite phenomena: light and darkness.

The next important step was taken by the physiologist E. Hering, who dedicated himself to the investigation of the sensations produced by color. He discovered phenomena that inspired his theory of the duplicity of colors, with which it was possible to explain numerous facts. However, the theory took a long time to acquire sufficient importance.

Hering's experiences are simple; we can all do them Let us suppose that a disk is observed, which, for example, has a red color; If you keep the view for several seconds and then close your eyes, you will see an image of the disk, but with a different color, in this case green. In the same way, if the observed disk is black, a white image will be seen: and if it were yellow, the image would be blue.

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In this way the famous trio of pairs of colors could be discovered: red-green, yellow-blue, and black-white. Hering assumed that in the process of vision each pair responded to a similar mechanism. He also affirmed that only these colors could be captured directly by our eyes; the others, however, were the result of the combination of those in varying proportions.

THE CELLS THAT ALLOW US TO SEE THE COLORS

It is evident that Hering's theory combines the physical properties of light with the physiology of our eye. It is really impossible to understand the vision of colors if they do not take into account its two fundamental aspects: one, light and its physical laws; another, the eye and its physiological functions. Let's see, then, this mechanism so perfected.

First of all we must take into account the following fact: we can only see those objects that are projected by light in the form of an image on the retina of our eye. This area of ​​the eye, sensitive to light and where images are formed as if it were the screen of a cinema, is called, as we have said, retina. It is made up of two different kinds of cells, each with different functions. They are known by the names of rods and cones; the first ones allow the vision when the light is not very intense, that is, in the semidarkness; the cones, on the other hand, act when the light is intense, as it happens during the day.

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What interests us for our question is that, of the two types of cells, only the cones are sensitive to colors, or, as it is also said, to chromatism. But will these cells have the ability to react in a different or independent way for each color? Not; we have already seen previously that each pair of opposite colors has in our vision the same process, and, as Hering distinguished only three pairs, it is evident that the cones must have three primary processes of reaction to the stimulus.

It is remarkable, then, how our eye, with only three different vision processes, makes it possible for us to participate in that immense range of colors that nature lavishes exquisitely in all directions.

PROTANOPOS, DEUTERANOPOS AND TRITANOPOS

It is interesting to remember that long before E. Hering, the researcher T. Young attributed to the retina the peculiarity of being sensitive to essentially three colors, and for that reason it was called the trichromatic theory of vision. For Young the colors were simple, and he only considered red, green and violet as physiological primaries.

Young's theory was adequate to explain the three known types of blindness for colors. Therefore, this fact turned out to be the most important proof of the validity of this theory.

People whose eyes can not normally perform the processes that allow to see the primary colors, will confuse them.

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We will mention the most important cases of such abnormalities. Protanopos are those individuals who suffer from blindness to red, which they generally confuse with green. When the green vision process is the one affected, they are called deuteranopos. The third possibility is that you suffer from blindness to violet. This case occurs very rarely, and those who suffer it are called tritanopos.

Hering's work was closely related to the physiology of vision. It is not uncommon, then, for them to become important when the investigations of another famous physiologist, the Russian scholar I. P. Pavlov, became known. However, these investigations related to issues beyond vision: Pavlov showed that in physiological processes, simultaneously with excitation, an opposite phenomenon occurs: inhibition. We immediately see something similar to what happened with the opposite colors, since they act in pairs. Thus, if yellow acts as an exciter, inhibition forms blue, and vice versa. Therefore, one of the general forms of functioning of the neuropsychic mechanisms had been discovered.

COMPLEMENTARY COLORS AND CONTRASTS

Suppose now that we have a green drawing, but so that we can superimpose it, first on a yellow background, and then on a blue background. We will check easily that in both cases the green will seem different.

We will see the drawing with a darker green when it is on the yellow background, by the aforementioned phenomenon of inhibition, since its opposite is blue, while, when it is on the blue background, we will see a lighter green, since the opposite of blue is yellow. We can all perform this experiment and check such a simple phenomenon.

Any theory that tries to explain the process of color vision based exclusively on the physical properties of light, has to face serious disadvantages, especially to explain this exceptionally interesting fact.

To complete our explanation, we must indicate that the colors are complementary that added to a mixture of the other two give the target.

Thus, for example, blue and orange (red + yellow) are complementary, since the sum of both gives us the feeling of white. The theory of complementary colors has its importance in painting, as well as the phenomenon of excitation and inhibition: a color stands out more if it is next to its complements.

SOME INTERESTING PROPERTIES OF THE NATURAL LIGHT AND THE ARTIFICIAL LIGHT

One of the most interesting properties of light is that of propagation through transverse vibrations. Let's see now what this means:

There are two types of wave motions: longitudinal and transverse. Example of the first is the sound: the molecules of the gaseous medium that transmit it vibrate moving in the direction in which the wave advances, that is in the longitudinal direction. As for the transverse wave movement, we will understand it if we imagine a calm lake in which a cork floats. When a stone is thrown on the smooth surface of the water, waves appear as concentric circles, and when they reach the cork they will move it up and down, that is, in the direction transverse to the direction of propagation. Hence the name that this movement receives. The oscillations of light are also transverse, but, unlike the cork of our example, which only moves from top to bottom, the light makes its oscillations in all possible directions, which are normal to the direction of propagation of the wave luminous

This is what happens with natural light, but now it is worth asking: will it be possible to obtain light that vibrates in only one direction? Yes, and the light that has this property is called "polarized light", which can be obtained by three procedures: the reflection, the double refraction and the simple refraction.

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It is interesting to see how polarized light is obtained. It is not difficult: it is enough to pass natural light through certain special materials, such as, for example, sheets of polaroid or tourmaline; the emerging light is already polarized.

Special instruments designed to obtain polarized light are called polariscopes or polarizers. It is very important, because of its simplicity and its great application, the so-called prism of Nicol, which is nothing more than a crystal of Iceland spar or calcite, carved in a suitable way for these purposes.

If one of the faces of Nicol's prism enters natural light, a beam of polarized light comes out of its opposite face, and what is extremely interesting is that if we rotate the prism, as if the ray of light were an axis, then also it rotates the direction in which the vibrations of the polarized ray of light are verified. So with Nicol's prism we not only get polarized light, but, more importantly, we can choose the direction of polarization.

A CURIOUS PHENOMENON: THE BIRTHFRINGENCE OF SOME CRYSTALS

There are certain crystals that have a very interesting and at the same time curious property, the so-called birefringence. When a beam of natural light crosses a birefringent crystal it is divided into two rays, so that two beams of polarized light come out of the crystal, and the remarkable thing is that the directions of polarization of each ray are not parallel but perpendicular. If you look at a figure with a birefringent crystal, you will see it double because of this property.

Let us now look at the following experience: let us suppose that we pass a ray of natural light through a prism of Nicol. As we know, polarized light will emerge from it according to a certain direction. What will happen if we now pass this light through another prism of Nicol? As the light vibrations are transmitted in a single direction, if this does not coincide with the polarization of the second prism, then the light can not pass. Indeed, only when the polarization directions of the two prisms are parallel, the light passes. This arrangement of two prisms, one after the other, is widely used, especially in devices such as polarimeters, saccharimeters, microscopes, polarizers, etc. The first prism is called polarizer, and the second, instead, analyzer.

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The use of polarized light in microscopes has given them a remarkable advantage: they allow to see the images in relief. In fact, microscopes that use natural light give flat images and do not show depth details; that is why most of the microscopes used for metallographic examinations, in mineral studies or in biology are polarizing, that is, they use light that has been polarized.

This stereoscopic effect has had other applications that, while not so important from the scientific point of view, are very interesting in other aspects; for example, the trials that have been done with the relief cinema. However, this cinema has not achieved success; This may be due to the fact that spectators must wear special glasses capable of polarizing the light, with which an effect similar to that of Nicol's prism is achieved, since through them each eye perceives an independent image, thereby achieving the stereoscopic illusion on relief of the figures on a screen